Traditionally, digitally controlled liquid patterning capability is accomplished by one of two technologies. In each technology, a patterning liquid is fed through channels formed in a printhead. Each channel includes a nozzle from which drops of liquid are selectively extruded and deposited upon a medium. When color marking is desired, each technology typically requires independent liquid supplies and separate liquid delivery systems for each liquid color used during printing.
The first technology, commonly referred to as “drop-on-demand” ink jet printing, provides liquid drops for impact upon a recording surface using a pressurization actuator (thermal, piezoelectric, etc.). Selective activation of the actuator causes the formation and ejection of a flying drop that crosses the space between the printhead and the pattern receiving media, striking the media. The formation of printed images or other patterns is achieved by controlling the individual formation of liquid drops, based on data that specifies the pattern or image.
Conventional “drop-on-demand” ink jet printers utilize a pressurization actuator to produce the ink jet drop at orifices of a print head. Typically, the pressurization is accomplished by rapidly displacing a portion of the liquid in individual chambers that supply individual nozzles. Displacement actuators are most commonly based on piezoelectric transducers or vapor bubble forming heaters (thermal ink jet). However, thermomechanical and electrostatic membrane displacement has also been disclosed and used.
U.S. Pat. No. 4,914,522 issued to Duffield et al., on Apr. 3, 1990 discloses a drop-on-demand ink jet printer that utilizes air pressure to produce a desired color density in a printed image. Liquid in a reservoir travels through a conduit and forms a meniscus at an end of an inkjet nozzle. An air nozzle, positioned so that a stream of air flows across the meniscus at the end of the liquid nozzle, causes the liquid to be extracted from the nozzle and atomized into a fine spray. The stream of air is applied at a constant pressure through a conduit to a control valve. The valve is opened and closed by the action of a piezoelectric actuator. When a voltage is applied to the valve, the valve opens to permit air to flow through the air nozzle. When the voltage is removed, the valve closes and no air flows through the air nozzle. As such, the liquid dot size on the image remains constant while the desired color density of the liquid dot is varied depending on the pulse width of the air stream.
The second technology, commonly referred to as “continuous stream” or “continuous” ink jet printing (CIJ), uses a pressurized liquid source which produces a continuous stream of liquid drops. This technology is applicable to any liquid patterning or selection application. Conventional continuous ink jet printers utilize electrostatic charging devices that are placed close to the point where a filament of working fluid breaks into individual drops. The drops are electrically charged and then deflected to an appropriate location by an electric field of self-image charge in a grounded conductor. When no drop deposition is desired at a particular location on the receiver medium, the drops are deflected into an liquid capturing mechanism, a drop catcher or gutter, and either recycled or discarded. When a print or pattern drop is desired, the drops are not deflected to the drop catcher and are allowed to strike the receiver media. Alternatively, deflected drops may be allowed to strike the media, while non-deflected drops are collected in the liquid capturing mechanism.
Conventional continuous ink jet printers utilize electrostatic charging devices and deflector plates that require addressable electrical components that must be very closely and precisely aligned to the continuous streams of patterning liquid without touching them. The patterning liquid, the liquid, must be sufficiently conductive to allow drop charging within a few microseconds. While serviceable, these electrostatic deflection printheads are difficult to manufacture at low cost and suffer many reliability problems do to shorting and fouling of the drop charging electrodes and deflection electric field plates. A continuous ink jet system that does not rely on drop charging would greatly simplify printhead manufacturing, and eliminate the need for highly conductive working fluids.
U.S. Pat. No. 3,709,432, issued to Robertson, on Jan. 9, 1973, discloses a method and apparatus for stimulating a filament of working fluid causing the working fluid to break up into uniformly spaced liquid drops through the use of transducers. The lengths of the filaments before they break up into liquid drops are regulated by controlling the stimulation energy supplied to the transducers, with high amplitude stimulation resulting in short filaments and low amplitudes resulting in long filaments. A flow of air is generated uniformly across all the paths of the fluid at a point intermediate to the ends of the long and short filaments. The air flow affects the trajectories of the filaments before they break up into drops more than it affects the trajectories of the liquid drops themselves. By controlling the lengths of the filaments, the trajectories of the liquid drops can be controlled, or switched from one path to another. As such, some liquid drops may be directed into a catcher while allowing other liquid drops to be applied to a receiving member. The physical separation or amount of discrimination between the two drop paths is very small and difficult to control.
U.S. Pat. No. 4,190,844, issued to Taylor, on Feb. 26, 1980, discloses a single jet continuous ink jet printer having a first pneumatic deflector for deflecting non-printing drops to a catcher and a second pneumatic deflector for oscillating printing drops (Taylor '844 hereinafter). A printhead supplies a filament of working fluid that breaks into individual liquid drops. The liquid drops are then selectively deflected by a first pneumatic deflector, a second pneumatic deflector, or both. The first pneumatic deflector has a diaphragm that either opens or closes a nozzle depending on one of two distinct electrical signals received from a central control unit. This determines whether the liquid drop is to be deposited on the medium or not. The second pneumatic deflector is a continuous type having a diaphragm that varies the amount a nozzle is open depending on a varying electrical signal received the central control unit. This deflects printed liquid drops vertically so that characters may be printed one character at a time. If only the first pneumatic deflector is used, characters are created one line at a time, being built up by repeated traverses of the printhead.
While this method does not rely on electrostatic means to affect the trajectory of drops it does rely on the precise control and timing of the first (“open/closed”) pneumatic deflector to create printed and non-printed liquid drops. Such a system is difficult to manufacture and accurately control. The physical separation or amount of discrimination between the two drop paths is erratic due to the uncertainty in the increase and decrease of air flow during switching resulting in poor drop trajectory control and imprecise drop placement. Pneumatic operation requiring the air flows to be turned on and off is necessarily slow in that an inordinate amount of time is needed to perform the mechanical actuation as well as time associated with the settling any transients in the air flow. Further, it would be costly to manufacture a closely spaced array of uniform first pneumatic deflectors necessary to extend the Taylor '844 concept to a plurality of closely spaced jets.
U.S. Pat. No. 5,963,235 issued to Chwalek, et al., on Oct. 5, 1999 discloses a continuous ink jet printer that uses a micromechanical actuator that impinges a curved control surface against the continuous stream filaments prior to break-up into droplets (Chawlek '235 hereinafter). By manipulating the amount of impingement of the control surface the stream may be deflected, along multiple flight paths. While workable, this apparatus tends to produce large anomalous swings in the amount of stream deflection as the surface properties are affected by contact with the working fluid.
U.S. Pat. No. 6,509,917 issued to Chwalek et al., on Jan. 21, 2003, discloses a continuous ink jet printer that uses electrodes located downstream of the nozzle, closely spaced to the unbroken fluid column, to deflect the continuous stream filament before breaking into drops (Chawlek '917 hereinafter). By imposing a voltage on the electrodes drops may be steered along different deflection paths. This approach is workable however the apparatus prone to electrical breakdown due to a build up-of conductive debris around the deflection electrodes.
U.S. Pat. No. 6,474,795 issued to Lebens, et al., on Nov. 5, 2002 discloses a continuous ink jet printer that uses a dual passage way to supply fluid to each nozzle (Lebens '795 hereinafter). One fluid passageway is located off-center to the nozzle entry bore and has a micromechanical valve that regulates the amount of flow that is supplied. The off-center flow from this passageway causes the jet to be emitted at an angle. Thus by manipulating this valve, drops may be directed to different deflection pathways. This approach is workable however the printhead structure is more complex to fabricate and it is difficult to achieve uniform deflection from all of the jets in a large array of jets.
U.S. Pat. No. 6,079,821, issued to Chwalek et al., on Jun. 27, 2000, discloses a continuous ink jet printer that uses actuation of asymmetric heaters to create individual liquid drops from a filament of working fluid and deflect those liquid drops (Chwalek '821 hereinafter). A printhead includes a pressurized liquid source and an asymmetric heater operable to form printed liquid drops and non-printed liquid drops. Printed liquid drops flow along a printed liquid drop path ultimately striking a print media, while non-printed liquid drops flow along a non-printed liquid drop path ultimately striking a catcher surface. Non-printed liquid drops are recycled or disposed of through a liquid removal channel formed in the catcher.
While the ink jet printer disclosed in Chwalek '821 works extremely well for its intended purpose, the amount of physical separation between printed and non-printed liquid drops is limited which may limit the robustness of such a system. Simply increasing the amount of asymmetric heating to increase this separation will result in higher temperatures that may decrease reliability.
U.S. Pat. No. 6,505,921 issued to Chwalek, et al. on Jan. 14, 2003, discloses and claims an improvement over Chwalek '821 whereby a plurality of thermally deflected liquid streams is caused to break up into drops of large and small volumes, hence, large and small cross-sectional areas (Chwalek '921 hereinafter). Thermal deflection is used to cause smaller drops to be directed out of the plane of the plurality of streams of drops while large drops are allowed to fly along nominal “straight” pathways. A uniform gas flow is imposed in a direction perpendicular and across the array of streams of drops of cross-sectional areas. This perpendicular gas flow applies more force per mass to drops having smaller cross-sections than to drops having larger cross-sections, resulting in an amplification of the deflection acceleration of the small drops. Such gas flow deflection amplification can provide needed additional separation between drops to be captured in a gutter versus drops that are allowed to deposit on a medium. However, to be effective, the apparatus of Chwalek '921 requires a substantial difference in large and small drop volumes which has the effect reducing printing speed as time and liquid volume is spent creating large drops.
U.S. Pat. No. 6,508,542 issued to Sharma, et al. on Jan. 21, 2003, also discloses and claims an improvement over Chwalek '821 that uses a gas flow to amplify the spatial separation between drops traveling along two diverging pathways, so as to improve the reliability of drop capture (Sharma '542 hereinafter). Sharma '542 teaches a gas flow that is emitted in close proximity to a gutter drop capture lip and that is generally opposed to both the nominal and thermally deflected flight paths of drops. The gas flow of Sharma '542 is illustrated as further splitting the drops into two pathways and is positioned so that the gas flow is losing convergence at a point where the thermally deflected drops are physically separating.
Effectively, the apparatus and method taught by Sharma '542 increases drop pathway divergence by reducing the drop velocity in the direction of the media and gutter. That is, by slowing the flying drops, more time is provided for the off-axis thermal deflection acceleration imparted at the nozzle to build up into more spatial divergence by the time the capture lip of the gutter is reached. The interaction of the gas flow of Sharma '524, and the diverging drop pathways, will also be very dependent on the time varying pattern of drops inherent in image or other pattern printing. Different drop sequences with be differently deflected, resulting in the addition of data dependent drop placement error for the printed drops. Further, the approach of Sharma '542 may be unsuitable to implement for a large array of jets as it is difficult to achieve sufficiently uniform gas flow behavior along a wide slit source so that the point of transition to incoherent gas flow would occur at the same distance from the nozzle for all jets of the array.
Notwithstanding the several inventions described above, there remains a need for a robust, high speed, high quality liquid patterning system. Such a system may be realized using continuous ink jet technology that does not rely on drop charging and electrostatic drop deflection. Further, such a system could be realized if sufficient drop deflection can be achieved to allow robust drop capturing without sacrificing print speed and pattern resolution by the formation of large volume drops or long flight paths from nozzle to medium. Finally, such a system requires simplicity of design that facilitates fabrication of large arrays of closely space jets.